When breeding fish there is produced materials which in high concentrations negatively will influence the growth balance in the surrounding medium, such as the sea or the lake. High concentrations appear upon breeding in smaller lakes, basins or at breeding in bays and near the seashores. The environmental pollution load will be a disturbance in direct proportion to the amount of fish raised and the water exchange ratio. The water exchange ratio increases exponentially very rapidly with an increased distance from e.g. a seashore and is at deep waters as big that such an emission of otherwise environmental pollution load increasing materials will be considered as very marginal. Hitherto the breeding of fish in open waters have mainly been carried out to an experimental extent. Selfsupporting breeding plants having volumes about 10,000 m.sup.2 are today built, but these plants cannot be located in open waters where the water exchange rate is satisfactorily without substantial reconstruction.
The breeding plants hitherto designed mainly have consisted of annular pontoons, which also are used as a working bridge and which carries one or more netted bags which by means of sinkers enclose a bigger or smaller water volume. The pontoons can be a rigid tubular structure or a number of floating bodies upon which bridges have been attached. These plants are however quite inappropriate in open sea, where waves several meters high can occur. It has also been suggested pontoons shaped as very coarse rubber hoses be used. Plants made in such a manner have proved also to stand very heavy sea but they have a negative effect which arises when the rubber tubes follow the movement of the waves, whereby the netted bag at irregular upwards and downwards movements will have a stressing influence on the fish.
A problem with big breeding plants adapted to open sea or lakes, which for instance can have a diameter of 40-50 meters, is the handling of the fish when taking up the fish, which has been brought about by lifting the netted bag, but the boarding and the transfer of loads such as fodder for the fish from a ship to the breeding plant can also, due to the rough sea, be a problem. Another problem at big breeding plants is that they preferably should be closable also at the top to prevent birds and other unauthorized creatures to reach the contents of the netted bag.